1. Field of the Invention
The present invention relates generally to semiconductor manufacturing. More specifically, the present invention pertains to monitoring characteristics of a processed substrate.
2. Description of related art
During semiconductor device manufacturing, a film (e.g., epitaxial silicon) is often formed or grown on a wafer surface. After the film is formed on the wafer, a characteristic of the film is measured. Surface quality of the film is one such characteristic that is measured to determine if a wafer is defective.
There are several methods to determine the surface film quality of the wafer. The light scattering method involves shining a bright light onto a wafer while a highly-skilled scientist visually examines the wafer to determine surface film quality. Another method also involves a subjective visual inspection of the wafer by one skilled in the art, but under an optical microscope. Thus, both of these methods suffer from their dependence on the subjective visual inspection of a fallible human being. A third method required bringing a processed wafer to a light scattering instrument that focused laser light on the processed wafer. The amount of laser light scattered by the wafer is quantifiable and is then used to determine the surface film quality of the wafer. Unfortunately, this third method requires a very expensive light-scattering tool that is separate from the processing tool that forms the film on the wafer. This third method uses a very costly off-line metrology. Thus, it is desirable to use a tool that is already being utilized to automatically indicate a characteristic, such as the surface film quality of a processed wafer.
The present invention relates to a computer-implemented method for automatically determining a characteristic of a substrate by using a film thickness monitor. In one embodiment, the film thickness monitor includes a computer system and is part of a larger processing tool. In another embodiment, the film thickness monitor and its computer system are part of an off-line metrology and are not part of a larger processing tool.
In one embodiment, the present inventive method comprises the following steps. A measured spectrum, which represents the reflectance of a sample substrate, is generated. A starting set of parameters (e.g., film thickness, substrate doping level) is chosen. A calculated spectrum is then generated according to the starting set of parameters. The difference, if any, between the measured and calculated spectra is calculated or determined. If the difference or residual error between the two spectra is not minimized, then at least one parameter is varied to generate a revised calculated spectrum. Typically, however, more than one parameter is varied (possibly more than once) before the residual error between the two spectra is minimized. The measured spectrum is then compared to the revised calculated spectrum to determine if the difference or residual error is minimized.
If the difference or residual error between the two spectra is minimized, then it must be determined whether the two spectra match well. This is because the fact that the residual error is minimized does not necessarily mean that the two spectra match well. In one embodiment, if the two spectra do not match well, which usually means there is a residual error greater than a predetermined acceptable residual error value, then the sample substrate probably has a defect or problem. Further tests can be run to pinpoint the exact cause or nature of the problem. The problem can typically be haze or poor surface quality of the sample substrate.
On the other hand, if the two spectra match well or correlate, then the sample substrate is probably acceptable and generally has the characteristics of the parameters used to generate the calculated spectrum. In other words, the residual error between the two spectra falls within an acceptable range and indicates a xe2x80x9cgood fitxe2x80x9d between the two spectra. Thus, the present invention uses a film thickness monitor to quickly and automatically determine if a sample substrate has a defect without depending on an additional and costly instrument or on a fallible human being.